A client/server implementation approach in which any platform may
act as a client or server or both is called:

A.
Peer-to-peer

B.
Simple file transfer

C.
Graphical User Interface (GUI)

D.
Application Programming Interface (API)

Explanation:
In answer Simple file transfer, a workstation or PC uses terminal emulation software
and a client application program to receive data from a host machine.
For answer API, an API defines how the client and server appear to each
other and supports the exchange of information without either entity
knowing the details of a particular resource that is accessed using the
API. One example is the Generalized Security Application Programming
Interface (GSAPI) that applications can use to access
security services.
Answer GUI, the GUI approach, is similar to the API
implementation and employs a user interface such as SQL to access a

server database.